<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
        margin: 0;
        padding: 0;
        list-style: none;
        }
        html,body{
        height: 100%;
}
        .wrap{
            height: 100%;
            background-color: black;
            position: relative;

        }
        .breathe{
            height: 80px;
            width: 80px;
            border-radius: 50%;
            background-color: #fff;
            position: absolute;
            left: 50%;
            top: 50%;
            margin-left: -40px;
            margin-top: -40px;
            box-shadow: 0px 0px 20px rgb(238, 255, 0);
            animation: change 3s infinite linear alternate;
        }
            /* 透明度 取值0-1*/
            /* opacity: 0.5; */
            @keyframes change {
                from{
                    opacity: 1;
                }
                to{
                    opacity: 0.5;
                }
        }
    </style>
</head>
<body>
    <div class="wrap">
        <div class="breathe"></div>
    </div>
</body>
</html>